Carboxyl group-containing vinyl resins, vinyl resins having an epoxy group and a hydroxyl group and curable resin compositions of an acid-epoxy base containing a curing catalyst have already been known, and used as the above carboxyl group-containing vinyl resin is a resin obtained by copolymerizing a carboxyl group-containing polymerizable monomer with a polymerizable monomer having an acid anhydride group to obtain a vinyl resin and semi-esterifying the acid anhydride groups contained in the vinyl resin with monoalcohol (refer to, for example, U.S. Pat. No. 5,508,352 or European Patent Publication No. EP-A-667,371).
The present inventors have investigated in detail the performances of the curable resin composition described above, and it has become clear that a coating film formed from the above resin composition is excellent in an acid resistance, a coating film hardness and a scratch resistance but there is the defect that a storage stability and an electrostatic coating property of the composition and a heat yellowing resistance of the coating film are still poor in terms of practical use.
An object of the present invention is to solve the defect described above without reducing the performances of a coating film such as an acid resistance, a coating film hardness and a scratch resistance in a conventional acid-epoxy base curable resin composition.
Intensive researches repeated by the present inventors have resulted in finding that the object described above can be achieved by using as a carboxyl group-containing resin, a specific acryl resin having a carboxyl group originating in a carboxyl group-containing polymerizable unsaturated monomer selected from acrylic acid, methacrylic acid, maleic acid and itaconic acid and a carboxyl group originating in a carboxyl group-containing polymerizable unsaturated monomer having a polymerizable carbon-carbon double bond and a carboxyl group separated from the double bond by at least 6 carbon atoms, and they have come to complete the present invention.